1. Field of the Invention
The present invention relates to a semiconductor nanoparticle having high luminescence properties and a method for synthesizing the same. Moreover, the present invention relates to a fluorescent reagent and an optical device comprising such semiconductor nanoparticle.
2. Background Art
Semiconductor nanoparticles with particle sizes of 10 nm or less are located in the transition region between bulk semiconductor crystals and molecules, so that they exhibit physicochemical properties different from those of either bulk semiconductor crystals or molecules. In such region, the degeneracy of energy bands that is observed in bulk semiconductors is removed and the orbits become discrete, so that a quantum size effect appears in which the energy width of the forbidden band changes depending on particle size. According to the appearance of the quantum size effect, the energy width of the forbidden band of a semiconductor nanoparticle decreases or increases in response to an increase or decrease of particle size. This change of the energy width of the forbidden band affects the fluorescence properties of the particle in question. A particle that has a smaller particle size and wider forbidden band energy width tends to have a shorter a fluorescent wavelength, while a particle that has a larger particle size and a narrower forbidden band energy width tends to have a longer fluorescent wavelength. That is, it is possible to create a desired fluorescent color by controlling particle size. In addition to the properties described above, semiconductor nanoparticles have high durability against excitation lights, etc., and a region which can be excited widely extends more towards the shorter wavelengths than the fluorescent wavelength, so that simultaneous excitation of multiple fluorescent colors is also possible by using a single excitation light source. Thus, semiconductor nanoparticles serving as fluorescent material are gaining significant attention. Specifically, the fields related to biotechnology and to optical device technology are listed as fields in which semiconductor nanoparticles have been used actively, and further applications are expected in the future.
In order to use semiconductor nanoparticles serving as fluorescence material, it is desired that such particles have fluorescence properties in which the fluorescence spectrum has a waveform with a narrow and sharp full width half maximum (FWHM). Thus, it is necessary that the band gap fluorescence properties in response to the forbidden band widths of the semiconductor nanoparticles are made effective. However, even if a prepared bulk particle has a monodisperse particle size, such particle per se does not exhibit sufficient band gap fluorescence properties. As a reason for this, the presence of the energy level existing mainly at the surface site of the semiconductor nanoparticle is mentioned, and, since the energy level exists in the forbidden band inside the particle, it has been thought that the band gap fluorescence properties are inhibited. Due to the reasons mentioned above, the inactivation of the aforementioned energy level and the obtaining of the band gap fluorescence have become significant subjects.
A method for providing a solution to this subject relates to a (CdSe)ZnS semiconductor nanoparticle, which has a so-called core-shell type structure. The aforementioned method involves obtaining high luminescence properties by coating the semiconductor nanoparticle (CdSe) with a second semiconductor material (ZnS), which has a wider forbidden band width than that of the particle, and removing the energy level in the forbidden band of the particle, thereby making the band gap fluorescence properties effective. (JP Patent Publication (Kohyo) No. 2001-523758 A and J. Phys. Chem. B. 101:9463 (1997))
In addition, by achieving particle size monodispersion in an aqueous solution and carrying out particle surface reforming, inventors have been studying a method for making band gap fluorescence effective. As a result of intensive studies carried out by the inventors, a method for obtaining semiconductor particles having commercially adequate fluorescence properties has been developed, in which semiconductor nanoparticles synthesized by a size-selective photoetching technique are treated in a refining process, the particles are subjected to surface reforming using sodium hydroxide or amine-ammonium compounds, and the energy levels at the particle surfaces are made inactive by arranging the electron-releasing groups on the surfaces, such that the band gap fluorescence properties are made effective. Moreover, by coating the obtained nanoparticles with organic compounds such as one composed of amphiphilic molecules, we succeeded in obtaining semiconductor nanoparticles having improved chemical durability. According to a series of these methods, synthesis of semiconductor nanoparticles that have high luminescence properties was realized by using a safe and simple technique in an aqueous solution. The nanoparticles per se have sufficient durability. In addition, high durability can be imparted to them by allowing preferably usable organic compounds, such as amphipathic molecules, to bind to each other. However, for the purpose of synthesizing high-functional semiconductor nanoparticles by a more convenient method, the inventors have arrived at the present invention.